A Ni-based superalloy which includes many alloy elements such as Al and Ti and is a γ′ (gamma prime) phase-precipitation strengthened type is used as a heat resistant member for aircraft engines and gas turbines for power generation. The Ni-based superalloy is mainly configured by a γ phase (matrix) which is a Ni solid solution and a γ′ phase (precipitate phase) which is an L12 type intermetallic compound Ni3 (Al, Ti). In order to improve engine efficiency, it is effective that a turbine is operated at an extremely high temperature. For this, it is necessary that a durable temperature of each turbine member is set to be high. In order to increase the durable temperature of a Ni-based superalloy, it is effective that the amount of the γ′ phase is increased. Thus, an alloy having a large amount of the precipitated γ′ phase is used in a member requiring high strength. In addition, a turbine member used in a rotation component or the like requires high fatigue strength in many cases. In this case, hot working is further performed on a cast structure in a state where an alloy is melted and solidified, and thus recrystallization is accelerated. Then, a recrystallization structure in a state where a grain size of the matrix (base) is homogeneous and fine is obtained, and thus a substance which can endure a practical use environment is obtained for the first time.
From a viewpoint of performing hot working on the Ni-based superalloy up to having a predetermined shape, the amount of the γ′ phase is limited. If the amount of the γ′ phase which is a strengthening phase is too much, deformation resistance is increased and hot ductility is decreased, and thus susceptibility to cracks of a material in a hot working process is increased. Thus, the additive amount of a component such as Al or Ti, which contributes to strengthening is generally limited in comparison to a cast alloy which is obtained without hot working.
As the representative of a turbine member in which fatigue strength is practically gave weight, a turbine disk, a turbine case, a shaft, and the like are exemplified. All of the members have large or long product dimensions. Thus, in order to produce materials thereof with high efficiency and high yield, it is desirable that hot working is performed by applying high-speed hot working machines which are represented by a high-speed forging machine, a ring rolling mill, and the like, in accordance with a shape of a product. These high-speed hot working machines perform hot working with a small number of times of heating for a short working time in comparison to a free forging press machine which is industrially used as with the high-speed hot working machine. Thus, it is possible to obtain a predetermined shape with high efficiency.
In a case of such a high-speed hot working machine, a predetermined working amount is obtained for a shorter working time. As a result, a strain rate when a material is deformed is increased. Since an increase of the strain rate in hot working causes deformation resistance of the Ni-based superalloy to be increased, hot ductility is significantly decreased. If a high-speed forging machine or a ring rolling mill is used, hot working is performed at a strain rate higher than three times that in a case of using a free forging press machine.
When hot working is performed on a metal material in a high temperature zone, deformation resistance or hot workability varies depending on the size of the strain rate. If the strain rate is high, the deformation resistance tends to be increased and the hot ductility tends to be decreased. This is because, as the strain rate becomes higher, recovery as a thermal activation procedure does not occur and working hardening significantly occurs by high dislocation density during working. Further, in a case where an alloy having a large amount of the γ′ phase is worked at a high strain rate, the γ′ phase hinders moving of dislocation. Thus, larger working hardening is shown. Therefore, as the amount of the γ′ phase becomes more, hot ductility of a superalloy of a γ′ phase precipitation type is decreased at a high strain rate.
From such a circumstance, in a case where hot working is performed on an alloy having a large amount of the γ′ phase by using a high-speed hot working machine or a ring rolling mill, susceptibility to cracks of a material is higher than that in a case of using a free forging press machine and thus working is difficult. In practice, a superalloy to which a high-speed hot working machine or a ring rolling mill can be applied has limited types in comparison to those of a free forging press.
In a hot working process which is practically forging or rolling, heat is dissipated toward an outside air in contact with the surface of a hot working material or a die or a roll as long as a special heat-retaining mechanism is not provided around the hot working machine. Thus, the surface temperature is decreased along with an increase of a hot working time.
In a case where hot working is performed on the Ni-based superalloy with decreasing the surface temperature, the γ′ phase which is sequentially precipitated with the decrease of the temperature prevents moving of dislocation. Hot ductility is significantly decreased in comparison to the decrease of the temperature in a case of steel or the like for a general structure. This is because, if the temperature is decreased in a precipitation temperature zone of the γ′ phase, the amount of the precipitatable γ′ phase is increased from a thermodynamic viewpoint. The amount of the γ′ phase is increased by precipitating the large amount of the γ′ phase in the vicinity of the surface with heat dissipation. However, from a viewpoint of a precipitation hardening mechanism, as the amount of the precipitated γ′ phase is increased and the size of the γ′ phase is reduced, the γ′ phase causes the deformation resistance to be increased and causes ductility to be decreased. Further, the dimensions of the γ′ phase precipitated during cooling or the amount of the precipitated γ′ phase largely depends on a cooling rate. However, the γ′ phase in a case where cooling is performed at a rate of the degree of natural cooling in the air, the γ′ phase is very fine and the amount of the γ′ phase is large.
From such a circumstance, when the Ni-based superalloy which has a large amount of the γ′ phase and has high strength is worked without an occurrence of cracks in a material, an advanced hot working technology is generally required. Various efforts, for example, introduction of a transporting facility for ending working for a short time or a heat-retaining mechanism that suppresses the decrease of a temperature of a working material, in addition to selection of a suitable heating temperature are made. However, the type of a Ni-based superalloy on which hot working can be stably performed is limited.
Thus, a viewpoint of material strength of the Ni-based superalloy and a viewpoint of hot workability generally have a trade-off relationship. In particular, in the current situation, a Ni-based superalloy to which a high-speed hot working machine or a ring rolling mill as described above can be applied is limited to an alloy having a small γ′ amount. In a case of a Ni-based superalloy which requires good hot workability even though high-temperature strength of a product is slightly impaired, an alloy design as follows is made. That is, considering that Al, Ti, or other strengthening elements are reduced, and thereby the γ′ amount is reduced and the γ′ solvus temperature is decreased, and a melting point of a crystal grain boundary is not decreased, the alloy design is made such that a γ single phase region in which hot ductility is good in a high temperature zone is widened and hot working is performed in a γ single phase region in which the γ′ phase that strongly hinders deformation during hot working is not provided.
If the representative Ni-based superalloy is used as an example, the followings can be described.
As the representative of a γ′ phase precipitation strengthened type Ni superalloy which has relatively high strength and excellent hot workability, there is Waspaloy. This alloy has a low γ′ solvus temperature and a wide γ single phase region in a high temperature zone. Thus, hot working can be relatively easily performed in the γ single phase region and the hot working process at a high strain rate, as described above, can be performed.
As a Ni-based superalloy having strength higher than Waspaloy (Waspaloy® is a registered trademark of United Technologies Corporation), Udimet720Li (Udimet® is a registered trademark of Special Metals Co., Ltd.) is exemplified. This alloy has the amount of precipitated γ′ and the γ′ solvus temperature which are higher than that of Waspaloy, and is one of Ni-based superalloys on which performing hot working is most difficult. Since such an alloy has many added elements, a partial melting temperature is low and it is not possible to stably perform hot working in a temperature zone of the γ′ solvus temperature or higher. Accordingly, when hot working is performed on this alloy, working is necessarily performed in a coexistence zone of the γ phase and the γ′ phase. Hot working by a free forging press machine is possible, but hot working is very difficult because the γ′ phase hinders deformation. Therefore, in the current situation, the hot working process of a high strain rate, which uses ring rolling or the like is not actively used.
As a superalloy having strength much higher than Udimet720Li, there is an alloy of high Co and high Ti as disclosed in Patent Document 1. Similar to Udimet720Li, this alloy is an alloy which can be produced by a hot working process in the related art. However, since the amount of precipitated γ′ and the γ′ solvus temperature are equal to or more than those of Udimet720Li, this alloy is an alloy on which hot working is difficult to the extent which is equal to or more than that for Udimet720Li.